Pumps for incompressible fluids, such as oil, are often variable capacity vane pumps. Such pumps include a moveable pump ring, which allows the rotor eccentricity of the pump to be altered to vary the capacity of the pump.
Having the ability to alter the volumetric capacity of the pump to maintain an equilibrium pressure is important in environments such as automotive lubrication pumps, wherein the pump will be operated over a range of operating speeds. In such environments, to maintain a comparatively equilibrium pressure it is known to employ a direct, or indirect, feedback supply of the working fluid (e.g. lubricating oil) from the output of the pump to a control chamber adjacent the pump control ring, the pressure in the control chamber acting to move the control ring, against a biasing force, typically from a return spring, to alter the capacity of the pump.
When the pressure at the output of the pump increases, such as when the operating speed of the pump increases, the increased pressure is applied to the control ring to overcome the bias of the return spring and to move the control ring to reduce the capacity of the pump, thus reducing the output volume and hence the pressure at the output of the pump, to continue to maintain a comparatively equilibrium pressure despite the change in operating conditions, (speed).
Conversely, as the pressure at the output of the pump drops, such as when the operating speed of the pump decreases, the decreased pressure applied to the control chamber adjacent the control ring allows the biasing force, typically from a return spring, to move the control ring to increase the capacity of the pump, raising the output volume and hence pressure of the pump, to continue to maintain a comparatively equilibrium pressure despite the change in operating conditions. In this manner, a comparatively equilibrium pressure is obtained at the output of the pump over a range of operating conditions (speeds).
The equilibrium pressure is determined by the area of the control ring against which the working fluid in the control chamber acts, the pressure of the working fluid supplied to the chamber and the bias force, typically generated by the return spring and the characteristics of the hydraulic system that the pump operates within.
Conventionally, the equilibrium pressure is selected to be a pressure which is acceptable for the expected operating range of the engine and is thus somewhat of a compromise as, for example, the engine may be able to operate acceptably at lower operating speeds with a lower working fluid pressure than is required at higher engine operating speeds. In order to prevent undue wear or other damage to the engine, the engine designers will select an equilibrium pressure for the pump which meets the worst case (for example, high engine load or operating speed) conditions. Thus, at lower speeds, or lower engine loads, the pump will be operating at a higher capacity than necessary, wasting energy pumping the surplus, unnecessary, working fluid through the hydraulic system.
It is desired to have a simple variable capacity vane pump that can provide at least two equilibrium pressures in reasonably compact pump housing. Some prior art solutions use a dual spring configuration, as shown for example in WO2013049929 A1. It may be desirable to achieve similar benefits by using simple hydraulic connections, without the need for additional components.